fault line

The surface trace of a fault — the line along the ground where the fracture reaches the surface, or nearly does. Fault lines are often invisible, buried beneath soil and vegetation, their presence inferred from offset stream channels, lines of springs, scarps, and the alignment of earthquake epicenters. Some are unmistakable: the San Andreas fault line can be traced from the air as a scar running the length of California, marked by sag ponds, offset fences, and valleys that follow the fracture with eerie precision.